1. Field of the Invention
The present invention is concerned with a method and apparatus for scanning a communications channel for an available channel slot for use in communication. The invention can be employed particularly, but not exclusively in cognitive radio, spectrum agile radio, 802.22 WRAN and any general channel tuning radio.
The invention is generally concerned with a searching mechanism for identifying a “nearest free channel” in a dynamically varying spectrum.
2. Discussion of Background
In cognitive radio, spectrum agile radio or 802.22 WRAN, portions of spectrum are regularly or permanently unused by the primary target market, and are therefore available for use by a secondary market, as noted in “The Spectrum Framework Review” (OFCOM, November 2004: available at www.ofcom.org.uk/consult/condocs/sfr/sfi), “Spectrum Policy Task Force Report” (Technical Report, FCC, ET Docket 02-135, November 2004) and “Facilitating opportunities for flexible, efficient, and reliable spectrum use employing cognitive radio technologies” (FCC, ET Docket 03-108, December 2003).
In the context of Cognitive radio or the so called Secondary market mechanism, two sets of users can be considered, namely:                the Primary User (PU)—the licensed user of the spectrum or a user recognised as having high priority for the spectrum band, and        the Secondary User (SU)—an opportunistic user or “cognitive” user who accesses spectrum on a temporary basis when PUs are not making use of the spectrum.        
An SU node is allowed to access the spectrum when the primary or licensed user is not using the spectrum. This is on the basis that the SU node will cease use of the spectrum when operation of the PU commences. In a distributed cognitive radio network, a radio defining an SU node scans and searches the designated range of spectrum for a free channel during reconfiguration channel switching or initial transmission. Once a suitable free channel is found, the radio switches to this channel for transmission. Conventionally, radio channel scanning and searching is done unidirectionally, either in a direction of increasing frequency or of decreasing frequency, or is a priori programmed (controlled by a base station (BS)) to select a channel. Cognitive Radio is assumed to have the capability of changing the operating frequency or channel efficiently. A generalised graph search bidirectional algorithm is described in “Artificial Intelligence: A Modern Approach” (S. Russell, P. Norvig, 2nd Edition, Prentice Hall, 2003), “Unidirectional and Bidirectional Search algorithm” (P. Nelson, Software, March 1992) and “Bidirectional Heuristic Search Reconsidered” (H. Kaindl, G. Kainz, Journal of Artificial Intelligence Research, pages 283-317, 1997). Each of these discloses an A* search algorithm used in artificial Intelligence (AI).
The reader will appreciate that the use of this concept of division between PU and SU user devices is for the purpose of describing the present invention clearly with regard to the prior art, and an actual implementation of cognitive radio could be provided without this distinction being made, either explicitly or implicitly. Indeed, as described later, this distinction between PUs and SUs is not an essential element of the claimed invention.
Reliably sensing wide range of spectrum is a crucial issue in Cognitive radio networks. Cognitive radio related papers such as “Implementation issues in spectrum sensing for cognitive radios” (D. Cabric, S. Mishra and R. W. Brodersen, Asilomer conference on Signals, systems and computers, November 2004) and “Next Generation/dynamic spectrum access/cognitive radio wireless networks: A survey” (I. Akyildiz, W. Lee, M. Vuran and S. Mohanty, Elsevier journal of Computer networks, September 2006) analyse various channel sensing and detection methods such as matched filter detection, energy detection (radiometry), and cyclostationary detection.
Other papers on channel detection “Collaborative spectrum sensing for opportunistic access in fading environment” (A. Ghasemi, E. Sousa, IEEE DySPAN, November 2005) and “Cooperative sensing among cognitive radios” (S. Mishra, A. Sahai and R. Brodersen, IEEE ICC, June 2006) analyse how collaborative sensing improves detection in a fading environment.
None of the above papers discuss the channel scanning or searching methods but Akyildiz et al. stresses the importance and need for finding or selecting a channel quickly.
“A comparison of channel scanning schemes for distributed network formation and reconfiguration” (A. O. Mahajan, A. J. Dadej, K. V. Lever, Springer journal of wireless networks, June 1998) discusses channel scanning schemes but analyses synchronous scanning (round robin) and Asynchronous scanning (random scanning).
U.S. Pat. No. 5,613,208 discloses, in general terms, a method of scanning a channel in a cellular system. KR20010058553, KR960004811B and U.S. Pat. No. 4,977,611 describe further application of channel scanning in portable radio apparatus.